As the laser industry has evolved, the quest for optimizing the performance of existing lasers has intensified. There is a strong need for highly stabilized lasers in many fields. Investigators are particularly desirous of obtaining lasers which have stable power outputs as well as stable pointing or alignment characteristics. This stability is required both for direct experimentation as well as in the situation wherein the laser is used as an optical pumping source for another laser.
Some solutions to these issues are addressed in U.S. Pat. No. 4,939,739, issued Jul. 3, 1990 and incorporated herein by reference. The latter patent discloses a system for varying the alignment of a laser beam in order to maximize an operating parameter of a laser. In the embodiment disclosed therein, the power of the laser is optimized by varying the angle of one of the resonator mirrors which, in turn, varies the alignment of the laser beam. In another embodiment, variations in the position of the laser beam are corrected by varying the angle of one of the resonator mirrors.
The subject disclosure covers additional developments for position stability of a laser beam. One development disclosed herein is suitable for optically pumped solid state lasers. Referring to FIG. 1 there is illustrated a schematic diagram of a typical solid state laser 10. The laser 10 includes a resonator including convex mirror 12 and a concave mirror defining the output coupler 14. A solid state gain medium 16 (such as an Nd:YAG rod) is located within the resonator. A means, such as a flashlamp 18 or arc lamp, is provided to optically excite the gain medium.
In the latter type of solid state lasers, the energy from the flashlamp will tend to heat the gain medium creating a thermal lens in the rod. FIG. 2 illustrates the optical equivalent of the laser of FIG. 1 wherein the rod is replaced with a representation of the thermal lens 20 created due to the heating of the rod. If the rod is uniform and it is heated uniformly, the lens will not disturb the path of the beam. However, in the real world, uniformity is difficult to achieve. Thus, in operation, varying thermal gradients will develop which will result in the optical center of this effective thermal lens being laterally displaced. As the lens is displaced (illustrated in phantom line in FIG. 2), the alignment of the beam will be changed. As can be seen, there is an angular shift in the beam between mirror 12 and the lens and a lateral displacement of the beam at the output coupler 14. During operation, the thermal gradients will vary such that these misalignments will also vary.
The type of lateral misalignment of a beam described above can be quite detrimental. For example, translation of the beam can cause the mode to fluctuate. Further, if the laser is mode-locked, translation variations will effect the phase, pulse width and peak power of the beam. If the laser beam is used for second harmonic generation, changes in position of the beam in the doubler material will cause power variations in the second harmonic output.
Another application where this misalignment can be of critical importance is when the laser is used as a pump source in a system where highly stable, ultra short pulses are being created. Such a system is described in U.S. Ser. No. 07/381,969 U.S. Pat. No. 4,998,254 filed Jul. 18, 1989 and incorporated herein by reference. As noted in the latter patent, in order to optimize short pulse performance, it is necessary to stabilize the pump laser. In the system disclosed in the latter patent, the power of the pump laser is accurately controlled. It has also been found that short pulse performance can be further enhanced if variations in the lateral alignment of the laser beam, induced by varying thermal lens effects in the gain medium can be minimized. One aspect of the subject invention addresses such a control system.
In the above discussed laser system, thermal lens effects create variations in the alignment of the beam that are primarily lateral in nature at the output coupler. In other laser systems, variations in operating parameters will create both lateral and angular variations in the alignment of the beam at the output coupler. Both of these types of alignment variations will result in variations in the position of the beam outside of the resonator. In any case where both alignment variations effect beam position, the mere monitoring of beam position will not reveal the extent to which either of these two alignment effects is causing the change in beam position. Accordingly, it becomes difficult to correct for both lateral and angular errors unless these effects are isolated. It is another aspect of the subject invention to provide and approach for isolating and then compensating for both angular and lateral alignment variations.